Field of the Invention
The present invention is directed to a swivelling device for pulling pump drive strings which may be under torque.
Downhole rod pumps, particularly rotary pumps such as progressive cavity pumps (PCP), and stroke pumps, are generally driven by a drive string which extends through a concentrically arranged production tubing string. The drive string is made up of a plurality of rods or tubes which are connected together end-to-end by rod boxes. Alternatively the drive string may consist of one continuous rod cut and pinned to the desired length.
Progressive cavity pumps generally include a stator affixed to the production tubing and a corkscrew-shaped rotor connected to and supported at the working end of the drive string. The drive string is connected at its top end to a smooth rod, called a polish rod, which allows an effective seal to be created between the outer production tubing and the inner drive string. The polish rod is attached to a drive unit which, in operation, rotates the drive string. Upon actuation of the pump by rotation of the drive string, fluids are forced to the ground surface through an annular space provided between the drive string and the production tubing.
In operation, often the working end of a drive string will become jammed by such things as accumulation of debris, too high viscosity of the surrounding medium, or obstructions in the pump hole. When this happens with a rotary pump the end of the drive string may stop rotating, but the polish rod and remaining rods of the drive string continue to rotate until the drive unit is stopped by overload sensors; then a braking system that is built into the drive unit controllably releases back spin. The rotation of the drive string without rotation of its end causes the accumulation of torque, through twisting of the drive string. The accumulated torque, which may be substantial, due to the length of the drive string, may be released by "back spin", spinning the drive unit backwards. Under normal circumstances, the braking system releases in a controlled manner, thereby allowing the release of the torque accumulated in the drive string by "back spin". However, if the braking system locks on solid, the accumulated torque may be released uncontrolled once a drive clamp is disconnected from the drive unit by picking the polish rod up a few centimeters. The back spin is then released in an uncontrolled manner. In conventional systems, to pull the drive string and release torque, the drive unit is disconnected and the polish rod attached to a pony rod, a rod elevator and a rod hook. The rod hook contains a swivel which allows the drive string to "back spin", for the release of accumulated torque. Such conventional rod pulling devices, using a rod hook connected to a polish rod by a rod elevator, are not symmetrical along the axis of rotation. This means that the known devices are prone to "wobble", which can lead to stress under the high angular velocities attained when back spin is released uncontrolled. The "wobble" is exacerbated by the fact that the rod elevator and the rod hook form a point of articulation which can bend under the centrifugal force of rotation, allowing the bottom of the hook and the top of the elevator to "swing out" from the axis of rotation. The point of articulation between the rod elevator and the rod hook is a common cause of failure. Conventional devices also have moving parts for attaching the elevator to the rod hook, in the form of a locking finger on a hook portion of the rod hook, which is usually spring biassed in a closed position. The moving parts complicate the manufacture and assembly of the device, and provide weak points for failure. These factors contribute to make conventional rod pulling devices extremely unstable under conditions of high angular velocities, such as are encountered in the release of back spin. The conventional devices may fail during use, leading to equipment damage and human injury when the components fly apart.
Pumps other than rotary pumps, such as a stroke pump, may become sanded in or stuck. If this situation occurs the only way to pull the pump rod string out of the hole is to strip it out. This is achieved conventionally by installing tubing tongs over the rod string. A device called a "back off tool" is then clamped to the rod string. The tubing tongs are then used to rotate the back off tool and rod string. The rod string then backs off at the weakest connection. The swivel is then removed and a rod hook and elevator are installed for pulling off the rods. If there is no tubing drain, the fluid inside the tubing (surrounding the rod string) must then be swabbed out down to the top of the remaining rods (i.e. the rods below the weakest connection). Once this is achieved the tubing is pulled out of the hole to the remaining rods. The back off procedure is repeated until all rods and tubing are removed from the well.
Because the rods are backed off at the weakest connection the sudden rotation of the rods and/or torque created to back the rods off often causes the rod to jump violently. On some occasions the elevator can jump out past a safety latch of the rod hook, dropping the elevator onto the tubing tongs, causing equipment damage or possible injury to the working crew. The torque generated to back the rods off varies from type of pump, for example PCP (which rotates to the right under normal conditions) to a rod pump on a pump jack which strokes up and down. The pump jacks are not usually as tight requiring less torque to back the rods off, but still require substantial torque. The sudden back off causes sudden and very quick back spin that usually lasts a few seconds.
The conventional method for backing off of a rod pump on a pump jack is therefore dangerous, and suffers the same drawbacks as mentioned above for backing off of a rotary pump.
The "flush by" is a frequent procedure with PCP pumps. PCP pumps may need to be flushed because of debris causing the rotor to become sticky or the fluid may become too thick or heavy for the pump to force it to the ground's surface. A flush by is achieved by picking the polish rod up a few centimeters to unlock the polish rod clamp from the drive unit. Fluid is then pumped down the annular space between the production tubing the drive string; this causes the drive string to rotate backward, i.e. "back spin". The back spin may be very fast, for long periods of time causing equipment to fly apart.